In the manufacture of evacuated glass envelopes, such as for rectangular flat panel display devices, a plurality of bounding sidewalls are provided with a pair of opposed overlying plates or panels including a front or face plate panel and a back or base plate panel. The sidewalls as well as the opposed front and base plates are all preferably formed of glass and are fusion sealed together such as by means of frit material. As shown in my copending patent application Ser. No. 965,588 filed Dec. 1, 1978, and now Pat. No. 4,304,803, the front and back panels of the housing structure or envelope for the flat panel display device overlie and extend outwardly beyond the bounding sidewalls of such structure, and are necessary for providing a convenient electrical circuit feed-through means which extend along the inner surface of the front and/or back panels and through the frit seal formed with the adjacent sidewall to provide electrical connections from the exterior of the housing to internally positioned electrical components.
The assembly of a completed flat panel display device requires at least five thermal cycles before the final assembled display bulb is completed, which include glass compaction, sidewall assembly sealing, sealing alignment fixtures and supports for component parts to the face and base plates, sealing the sidewall assembly to the face plate and base plate assemblies, and finally, an evacuation and bake cycle. During the sealing and bake-out cycles, the overhang regions of the face plate and base plate, which may overhang as much as about 1/2" or so about the entire perimeter of the sidewalls, are subject to heating and cooling from both sides. All other regions of the housing, on the contrary, are heated and cooled only on one side. Such difference with respect to the applied heat and cooling is the cause of substantial thermal gradients in the glass components.
The stresses induced by the thermal gradients are of primary concern during heating due to the fact that rates are maximized in order to achieve proper frit flow for the sealing operation, and to obtain increased process rates during bakeout. In this connection, the internal surface of the face and base plate regions are subjected to a tensile stress, and any discontinuities which may be located at the ends of modular supports or vane spacers within the housing structure serve as stress concentrators and initiate failure origins. The thermal gradients which are generated in the glass components due to the differential heating produced by heating both the inner and outer surfaces of the overhang portion, while only heating the outer surface of the remainder of the structure, tend to result in structural failures of the housing during the sealing and bake-out cycles.
The present invention reduces the problem of glass breakage occasioned in the sealing and bake-out cycles of a housing enclosure for a flat panel display device by utilizing shielding means for retarding the heating of the overhanging glass portions of the face plate and base plate so as to reduce stress and permit higher process rates.